The present invention relates to an optical communication system using an optical branching/coupling device.
In one example of a conventional optical communication system, as shown in FIG. 10, an office device accommodates subscriber devices that need a high speed and large capacity communication through exclusive optical fibers (transmission lines) respectively.
On the other hand, a PON(Passive Optical Network) system was developed and practically used in resent years. The PON system is suitable that an office device accommodates subscriber devices that need wide band communication at low cost.
The PON is, as shown in FIG. 11, provided with an optical branching/coupling element between the subscriber devices and the office device. The optical branching/coupling element is a passive device that does not need an electric source. The office device is connected to the optical branching/coupling element through a single optical fiber or a double optical fiber due to redundancy. Each of the subscriber devices is connected to the optical branching/coupling element through an exclusive optical fiber.
The optical branching/coupling element distributes a downstream optical signal from the office device toward each of the subscriber devices. And also, the element combines an upstream optical signal from the subscriber device toward the office device. As shown in FIG. 3, TDMA (Time Division Multiple Access) is used for the upstream communication in order to multiplex the upstream signal from the subscriber devices on the single optical fiber. TDM (Time Division Multiplexing) is used for the downstream communication in order to multiplex the downstream signal toward the subscriber devices.
The PON can reduce construction cost of the communication system compared with the on-to-one connection in FIG. 10 because of sharing the optical fiber between the office device and the optical branching/coupling element. Further, since the PON uses the passive device as the optical branching/coupling element, it improves the system reliability in maintenance compared with the system using a passive element to multiplex the optical signal.
However, each of the subscriber devices and the optical branching/coupling element are connected by the single optical fiber in the PON, it has a low system reliability in a resistance to a transmission line fault such as a disconnection of the optical fiber.
An optical ring network is known as the other type of the conventional optical communication system. The optical ring network includes an optical fiber arranged like a ring and data flow only one direction in the optical fiber. The subscriber devices are connected at any points to the transmission line. In such the optical ring network, since the data flow in one direction, the disconnection of the optical fiber or a cut off of the power supply of the node device results the system down due to stop of data flow. In order to avoid such the system down, a bypath function and/or a loopback function are usually prepared in the optical ring network.
FIG. 12 shows the bypath function. The bypath function forms the data flow along the route shown in a broken line without passing the node device. In the normal state, the data flow along the route shown in a solid line via the node device.
As shown in FIG. 13, any faults in the optical transmission line control the node devices, which are located with the fault portion between, to turn back the data flow and to form a new loop. It is the loopback function.
The ring network is provided with optical switches to exchange the optical fibers for the bypath function and/or the loopback function.
A conventional optical ring network is, for example, disclosed in Japanese laid-open patent publication No. Sho 57-1866855. The network disclosed in the publication employs an 1-to-n optical communication system that includes a center device, n pieces of remote devices and an optical fiber loop that connects the devices. The number n is an integer that equals to or is larger than 2. The center device switches the data transmitting mode using the optical switch. In a first mode, the center device transmits the data in one direction to the transmission line. In a second mode, the center device transmits the data in both directions via an optical branching element.
The remote devices pick up and receive the data (the optical signal) from the transmission line in spite of the direction of the data flow.
The disconnection of the optical fiber changes the transmission mode from the first mode to the second mode so that all of the remote devices are able to receive the optical data signal. However, since there is an one side communication from the center device to the remote devices in the conventional ring network, the center device cannot specify the fault portion in the transmission optical fiber.
The present invention is aim to provide an optical communication system, which includes an optical branching/coupling element, can certainly detect a transmission line fault such as a disconnection of an optical fiber.
According to an aspect of the present invention, an optical communication system includes:
an office device that includes a pair of optical transmitting/receiving devices for act and standby systems;
a subscriber line that is a ring network of an optical fiber of which one terminal is connected to the optical transmitting/receiving device of the act system and the other terminal is connected to the optical transmitting/receiving device of the standby system;
a plurality of optical branching/coupling elements that are passive elements arranged on the subscriber line;
a plurality of subscriber devices arranged corresponding to the optical branching/coupling element to be connected to the branched lines;
means for detecting faults generated in the subscriber line; and
means for controlling the optical transmitting/receiving devices, wherein the controlling device actuates the optical transmitting/receiving device of the act system in normal state and actuates both of the optical transmitting/receiving devices when the detecting means detect any faults.
It is desirable that the subscriber line is a single core optical fiber.
Preferably, the office device further comprises means for transmitting a test signal, each of the subscriber devices includes means for transmitting a response signal corresponding to the test signal and the detecting means specifies fault point based on the response signals from the subscriber devices.
In the preferred embodiment, the optical branching/coupling element comprises first, second and third optical couplers that are connected one another to form a triangle network. The first optical coupler distributes light from the act system between the connected subscriber device and the subscriber line at the standby system side, the second optical coupler distributes light from the standby system between the connected subscriber device and the subscriber line at the act system side, and the third optical coupler distributes light from the connected subscriber device between the act system and the standby system.
The distribution ratios of the first, second and third optical couplers may be determined based on the location of the connected subscriber device in the ring network.
If the distribution ratio of the connected subscriber device side to the subscriber line side at the first optical coupler is assumed as m:n, this ratios at the second optical coupler should be n:m, the distribution ratio of the act system side to the standby system side at the third optical coupler should be m:n. In such the case, increased distance from the optical transmitting/receiving device of the act system increases the value of m, but decreases the value of n.